High-grade lung adenocarcinoma with fetal lung-like morphology: Clinicopathologic, immunohistochemical, and molecular analyses of 17 cases

Low-grade lung adenocarcinoma of fetal lung type, which is well characterized by its unique clinicopathologic and molecular features, is recognized as a distinct variant of lung cancer. In contrast, high-grade lung adenocarcinoma with fetal lung-like morphology (HG-LAFM) has not been studied widely. To characterize this subset better, we analyzed 17 highgrade adenocarcinomas with at least focal component resembling a developing epithelium in the pseudoglandular phase of the fetal lung. These rare (ca. 0.4%) carcinomas occurred predominantly in elderly men with a heavy smoking history, who showed elevated serum α-fetoprotein in 4 of 5 cases tested. Histologic examination revealed a fetal lung-like component as a focal finding accounting for 5% to 60% of the total tumor volume. It was invariably admixed with tissues having a morphology not resembling that of a fetal lung. A coexisting non-fetal lung-like element was quite heterogenous in appearance, showing various growth patterns. However, clear-cell (88%), hepatoid (29%), and large cell neuroendocrine carcinoma (24%) histology seemed overrepresented. HG-LAFM was characterized immunohistochemically by frequent expression of α-fetoprotein (41%), glypican-3 (88%), SALL-4 (59%), neuroendocrine markers (82%), CDX-2 (35%), and p53 (65%). HG-LAFM was molecularly heterogenous in that EGFR or KRAS mutation was observed in 22% of cases tested for both. Our data indicate that HG-LAFMs might form a coherent subgroup of lung adenocarcinomas. However, the uniformly focal nature of the fetal lung-like element, widely diverse coexisting non-fetal lung-like histology, and inhomogenous molecular profiles lead us to believe that HG-LAFM is best regarded as a morphologic pattern showing characteristic association with several clinicopathologic parameters rather than a specific tumor entity.